pumps in series: control

[jrwchem]

I am interested to know/understand the standard control configuration for 2 pumps in series.

What i have seen is the first pump (fed from tank) has discharge valve controlled by tank level.

The second pump has its discharge valve controlled by suction pressure of the second pump.

Is it possible to control the system with only the first valve? (Someone said to me that it can't be done that way becuase the flow of the second pump wont match the first pump flow, and could result in draining the inbetween pipe- resulting in vacuum).

Of course part of the answer is that the suction pressure is controlled to prevent caviation in the second pump.


Any advice/comments?

jon

 

[JJPellin]

The only place we routinely run pumps in series in a refinery is crude from the tanks to the crude units. There is normally a primarily pump at the tank that pumps the crude to a set of crude booster pumps that pump it up to enough pressure to get into the desalters. In this service, the controls are done with a flow control valve downstream of the second pump. The first pump has no control. Since the first pump is a high flow, low head design, it simply puts up as much flow as the second pump demands. Pumps in series are more common on a pipeline. I am sure that there are some pipeline people who can address this better. The main difference with a pipeline is that they have very long runs between pumping stations that give the system enough pressure drop to hold the pumps back on their curves.

I would not advise throttling on the first pumps with no control on the second. As you say, the second pump will potentially draw down the interconnecting line to vacuum and cavitate the second pumps badly.


Johnny Pellin

 

[dcasto]

JJPellin is right on, the same is true on pipeline pumps.

 

[BigInch]

Pipelines: If the distance and/or elevations between pumps are substantial, as between pipeline pump stations, you might want to put each pump on their own suction pressure control. A minimum suction pressure set point can be used to control pump flow either with a discharge flow control valve, or possibly via control of each pump's rpm.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MortenA]

some high speed pumps (e.g. water injection pumps) have very high NSPH e.g. +8 barg - so cavitation (vacuum) could occur even at fairly high discharge pressure from the first pump.

It does however sound as if your two pumps could be better matched?

Still what does the intermediate valve do? If the flow rate is too high (e.i. the discharge pressure too low) then it closes the valve - but the 2. stage pump will not see an increase in suction pressure - this it throttled in the intermediate valve - so the main function of this valve seems to be to "starve" the 2. stage pump?

Best regards

Morten